Atrial fibrillation is a cardiac disorder where the heart's two small upper chambers (the atria) quiver instead of beating effectively. With atrial fibrillation, blood may not be pumped completely out of the atria, so that the blood may pool along the atrial walls, and eventually clot. If a blood clot in the atria leaves the heart and becomes lodged in an artery in the brain, a stroke may result.
Treatments for atrial fibrillation (AFib) include medications to decrease blood clotting, medications to slow down rapid heart rate associated with AFib and electric shock to restore normal heart rhythm when medications do not improve symptoms. Other techniques include surgery to disrupt electrical pathways that generate AFib and uses of atrial pacemakers to regulate the heart rhythm.
The heart beat has two main phases called “diastole” where the heart relaxes and fills with blood and “systole” where the heart contracts and pumps out the blood. The contraction of the heart muscle is caused by an electrical wavefront that typically starts in the so called “sinoatrial” (SA) node of the atrium and spreads over the two atria. The wavefront reaches the so called “atrioventricular” (AV) node. The AV node delays the signal to allow the atria to finish contracting in response to the electrical activation. The contraction of the atria helps move the blood from the atria to the ventricles.
From the AV node, the electrical signal spreads through the His-Purkinje system, fibers that form a specialized conduction system that quickly propagates the wavefront to all the regions of the ventricles, and causes the ventricles to activate and contract. The contraction of the ventricles pumps the blood into the lungs and the body. At the end of the cycle, the ventricles relax and the whole process repeats.
An electrocardiogram (ECG) is used to assess rhythm disturbances in the heart. The ECG measures electrical activity of the heart as reflected through electrical potentials produced at the body surface. In a medical setting, e.g., doctor's office or hospital, a standard ECG is obtained by placing 10 small electrodes on the patient's body in a specific pattern and recording 12 channels of ECG for a brief period of time. For longer-term ECG monitoring, 3 to 5 electrodes are typically used to obtain 1 or 2 channels of ECG signals.
The ECG signal typically is a repeating pattern of three relatively distinct waveform components. One component is the “P wave” which represents atrial depolarization, e.g., the wavefront generated as the electrical impulse from the sinoatrial (SA) node spreads throughout the atrial musculature. The P wave precedes a second component, the “QRS complex.” A “PR Interval” represents the time it takes an impulse to travel from the atria through the AV node, bundle of His, and bundle branches to the Purkinje's fibers. The PR Interval extends from the beginning of the P wave to the beginning of the QRS complex. The “QRS Complex” component represents ventricular depolarization. The QRS complex is a large waveform typically composed of three (3) waves, the Q wave, the R wave, and the S wave. The Q wave is at the beginning of the QRS complex. The Q wave may or may not always be present. The R wave is typically the first positive deflection and the S wave is the negative deflection that follows the R wave. The third component is the “T wave,” which represents the electrical recovery of the ventricles (the electrical recovery of the atria is usually buried in the QRS complex or T wave, or is too small to be seen). The time interval between two consecutive beats, the so-called beat interval, is often measured from R-wave of one beat to R-wave of the following beat. The measure between two consecutive R waves is called the RR interval.
The QRS complex is usually the dominant feature of an ECG. The P wave is much smaller than the QRS complex because the atria generate less electrical activity than the much more massive ventricles. Other components of an ECG include the “Q-T Interval” which represents the time necessary for ventricular depolarization and repolarization and extends from the beginning of the QRS complex to the end of a “T wave.” By analyzing the pattern of the ECG, medical professionals can gain insight into the condition of the heart.
In an ECG from a healthy heart with normal rhythm, with a non-noisy signal, the large QRS complexes are separated by a fairly flat signal, except for a small upright bump (the P wave) about 120-200 ms before the QRS complex. A P wave is “conducted” when the atrial electrical activity conducts through the AV node, causing electrical activation of the ventricles and the QRS complex. A P wave is considered “non-conducted” when it fails to lead to a QRS complex. Non-conducted P waves can occur because the P wave was premature, or because of the condition called AV block, or other reasons. P waves that are blocked due to AV block are said to be “blocked P waves”. By definition, at most one of the P waves in the RR interval is conducted and any other P waves in the same RR interval are non-conducted.
In atrial flutter, the atrial rhythm increases to approximately 250-350 beats per minute. The accelerated atrial rhythm is sometimes visible as continuous waves in the ECG, with several waves appearing in a continuous connected pattern in each RR interval, quite different from the normal pattern of a single P wave in each RR interval. These waves of continuous, cyclic atrial activity are called flutter waves or F-waves, and may form a sawtooth pattern. During atrial flutter, the ventricular response sometimes becomes locked into a regular pattern with the atrial activity, so that for example, every third flutter wave results in a QRS while the other flutter waves are not conducted. In other cases, conduction of the flutter waves is more random, resulting in an irregular ventricular rhythm.
As the rate increases over 350-400 beats per minute, the rapid atrial rhythm is called atrial fibrillation. Sometimes the atrial activity may be visible in the RR interval as continuous, cyclic activity referred to as “f waves,” or coarse atrial fibrillation. Typically, the “f waves” are cyclic, but not as organized or consistent in shape as the “F waves” of atrial flutter. When viewed in two ECG channels, the cyclic activity of the “f waves” may be seen to alternate back and forth between channels in what appears to be modulated electrical activity.
At other times, atrial fibrillation may be present with no obvious cyclic activity visible in the ECG, but with low amplitude disorganized “noise” in the baseline. In other cases, there may be total absence of atrial activity, suggesting that the fibrillation has become greatly disorganized.